Mechanism of a concentration-dependent switch between activation and inhibition of Arp2/3 complex by coronin.

Arp2/3 complex is a key actin filament nucleator that assembles branched actin networks in response to cellular signals. The activity of Arp2/3 complex is regulated by both activating and inhibitory proteins. Coronins make up a large class of actin-binding proteins previously shown to inhibit Arp2/3 complex. Although coronins are known to play a role in controlling actin dynamics in diverse processes, including endocytosis and cell motility, the precise mechanism by which they regulate Arp2/3 complex is unclear. We conducted a detailed biochemical analysis of budding yeast coronin, Crn1, and found that it not only inhibits Arp2/3 complex but also activates it. We mapped regions required for activation and found that Crn1 contains a sequence called CA, which is conserved in WASp/Scar proteins, the prototypical activators of Arp2/3 complex. Point mutations in CA abolished activation of Arp2/3 complex by Crn1 in vitro. Confocal microscopy and quantitative actin patch tracking showed that these mutants had defective endocytic actin patch dynamics in Saccharomyces cerevisiae, indicating that activation of Arp2/3 complex by coronin is required for normal actin dynamics in vivo. The switch between the dual modes of regulation by Crn1 is controlled by concentration, and low concentrations of Crn1 enhance filament binding by Arp2/3 complex, whereas high concentrations block binding. Our data support a direct tethering recruitment model for activation of Arp2/3 complex by Crn1 and suggest that Crn1 indirectly inhibits Arp2/3 complex by blocking it from binding actin filaments.